The prehydrolysis-sulfate (Kraft) cooking for the production of special pulps having a high content of alpha cellulose was developed in the 1930's, see e.g. Rydholm, S. E., Pulping Processes, pp. 649 to 672, Interscience Publishers, New York, 1968. The basic idea is to remove as much hemicellulose as possible from cellulose fibers in connection with delignification, so as to obtain a high content of alpha cellulose. This is essential because the various end uses of such pulps, dissolving pulp for instance, do not tolerate short-chained hemicellulose molecules with a randomly grafted molecular structure.
A separate prehydrolysis step permits the desired adjustment of the hydrolysis of hemicelluloses by varying the hydrolysis conditions. In the prehydrolysis-kraft cooking process the necessary delignification is not carried out until a separate second cooking step. The prehydrolysis is carried out either as a steam or water phase prehydrolysis, or in the presence of a catalyst. In the former “steam” processes, organic acids liberated from wood during the process establish the necessary pH conditions and perform a major part of the hydrolysis, whereas in the latter “water” process, small amounts of mineral acid or sulfur dioxide may be added to “assist” the prehydrolysis. In autohydrolysis the prehydrolysis stage is carried out without any addition of acids. Conventionally is autohydrolysis established at some 30-40° C. higher temperature than with acid addition.
Conventionally after prehydrolyzing the cellulosic material in a reactor, the hydrolysate and the prehydrolyzed cellulosic material are neutralized in the reactor with alkaline neutralizing liquor so as to produce neutralized hydrolysate and neutralized prehydrolyzed cellulosic material. There is hydrolysate both in the free liquid outside the chips and also trapped and immobilized inside the chips.
In Bio Pulping, as much as possible of the hydrolysate can be recovered before the neutralization step in order to be able to utilize the carbohydrates released in the prehydrolysis as an additional product from the mill. A separate washing stage, in which the digester is first filled up with a washing liquid and then the liquid containing the carbohydrates is displaced from the digester, can be used between the prehydrolysis and cooking stages.
Displacing the hydrolysate out through one end of the digester using a displacement liquid added at the other end, is the established method but has shown that the displacement front developed is nothing but perfect. Instead the perfect displacement, by an even liquid transition zone, is practically impossible to obtain as there are voids or areas in the chip volume with less packing, and especially close to the vessel wall. These voids cause the displacement liquid to penetrate faster and develop streaks of well displaced chip volumes and likewise volumes of less displaced chip volumes. As a result, the hydrolysate will become diluted with the displacement liquid sooner than expected compared with if a perfect displacement front was established.
U.S. Pat. No. 8,262,854 describes an improved method for treating lignocellulosic material, wherein the digester and its contents are first heated with direct steam to a predetermined hydrolysis temperature in a steam phase and then a small volume of washing liquid is introduced into the top of the digester which washing liquid is trickling down through the chips column and removed from the lower end of the digester. According to the process described in U.S. Pat. No. 8,262,854 the hydrolysate is recovered by utilizing trickle-bed type down-flow of hydrolysate. In this method the first fraction of the trickled-down hydrolysate is collected as a product fraction and the second fraction is discharged from the digester to a hot hydrolysate storage tank to be used as the first trickle flow liquid in the next batch. By the trickle-bed type recovery in two steps it is obtained a concentrated first hydrolysate, but the recovery step using trickling down is too slow and therefore it is disadvantageous to production capacity but also for the pulp quality. Additionally, the treatment is uneven to the contents of the digester; it is obvious that channeling will occur during the treatment, i.e. the liquid goes where it is easiest. The washing will thus be uneven trough the bed of material in the digester due to these channeling effects such that some volumes of the chip bed will be subjected to less washing. Hence, a large amount of hemicellulose rich hydrolysate may still be kept in the chip volume.
In EP 2430233 is disclosed another method to recover the hydrolysate from a steam phase prehydrolysis much quicker than that possible using the trickle down method as disclosed in U.S. Pat. No. 8,262,854. In EP 2430233 is water introduced into the digester after prehydrolysis at top and bottom and subjected to internal circulation while filling the digester. The water filling may be continued until the entire chip volume inside digester is drenched in water. The water with its content of hydrolysate is displaced by another liquid using conventional displacement and the process may continue by a neutralization cooking process known in the art.